In our previous guide, you had to install the FaviconizeTab extension along with PermaTabs to shrink tabs, keep them around between sessions, and prevent them from being inadvertently closed. With this extension installed, just Ctrl+Click the tab, select App Tab, and any tab will shrink down to an icon only on the left of your Firefox window. You can set it to retain your App Tabs between sessions and protect them from closing.

App Tabs is a free download, works with Firefox. It's currently experimental, so you may encounter some bugs. It worked without a hitch for me on Windows, but rendered kind of wonky in OS X, so your mileage may vary.